Privacy-preserving publish-subscribe protocol in a distributed model

ABSTRACT

A method and system for providing privacy in a publish-subscribe protocol is provided. A server transmits to a client a public key. The server receives from the client a pseudonym of an interest based on a division malleable commitment method applied to the public key, wherein the pseudonym of the interest functions as a commitment of the client. The server encrypts an item with a padded key and encrypting the padded key. The server transmits to the client, the encrypted item and a pseudonym of a topic associated with the item based on a modification of the commitment by the server using a hybrid conditional-oblivious transfer protocol. When the interest of the client equals the topic associated with the item, the client retrieves a correct padded key to decrypt the encrypted data item; otherwise the client retrieves a random key that is unable to decrypt the encrypted data item.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application No. 61/602,260 filed Feb. 23, 2012, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to publish-subscribe protocols. More particularly, the present invention relates to a method and system for providing privacy in a publish-subscribe protocol.

BACKGROUND OF THE INVENTION

Publish-subscribe protocols have been employed for the distribution of streaming data. A common publish-subscribe protocol is an RSS (Rich Site Summary) feed. An RSS feed is a family of web feed formats used to publish frequently updated works in a standardized format. The data transmitted in an RSS feed may include blog entries, news headlines, audio, and video. RSS feeds or documents include full or summarized text, plus metadata such as publishing dates and authorship. RSS feeds can be read using software called an “RSS reader”, “feed reader”, or “aggregator”, which can be web-based, desktop-based, or mobile-device-based. The user subscribes to a feed by entering into the reader a URI of the feed or by clicking a feed icon in a web browser that initiates the subscription process. The RSS reader checks the user's subscribed feeds regularly for new work, downloads any updates that it finds, and provides a user interface to monitor and read the feeds.

A user can subscribe to a topic, such as finance, and receive in an email daily or monthly or weekly messages only in finance. The user receives RSS feeds only in areas (associated with the topic) to which they subscribe. The user does not receive of all documents that are published by one particular publisher.

An RSS feed is a specific instance of the more general class of publish-subscribe protocols which employ a publish-subscribe architectural pattern. Publish-subscribe is a messaging pattern where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers, called subscribers. Instead, published messages are characterized into classes, without knowledge of what, if any, subscribers there may be. Similarly, subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what, if any, publishers there are.

More particularly, in a publish-subscribe architecture, a subscriber can specify interests-cats, dogs, the stock market, finance, education, etc. A publisher may periodically publish items (e.g. documents) that may include attached tags, known as topics. These topics are included in a dictionary of topics. The dictionary is shared with subscribers. The subscribers may find their interests in the dictionary. A dictionary is a collection of all topics that each item may or may not relate to, and is known to all participants (e.g., subscribers and publishers). Interests are elements from the dictionary associated with a subscriber. Topics are elements from the dictionary associated with an item. Items may be digital documents in any format. If one of the interests of the subscriber is determined by the publisher to be equal to one of the topics of the next item to be published by the server, then the subscriber receives the item once it is published by the publisher. If no interests match any topics in the dictionary of the publisher, then the subscriber does not receive the item to be published.

A problem often encountered in circumstance where publish-subscribe protocols are employed is privacy violations—e.g., privacy with respect to transmitted data and/or the interests and/or identity of the subscribers. In the examples below, the clients are not malicious and clouding but are considered honest but curious.

For example, in a typical RSS feed, a subscriber reveals their interests, e.g., finance, and the publisher may view the interests; thus, the publisher may obtain some information about the personal choices of the subscriber. As a result, the privacy of the subscriber may be violated. Other instances of violations of privacy are more sensitive. For example, from the government's perspective, there may be sensitive databases that reveal sensitive material and topics, e.g., an agency may publish documents. One agency is interested in a certain document; another agency may be interested in another document. In certain circumstances, without privacy protections in place, an intruder in one agency may determine the interests of another agency. In another example, one or more subscribers is interested in the Facebook stock. As a result, a publisher or an external intruder may learn that a number of subscribers are suddenly interested in Facebook stock. Thus, privacy is an important issue with respect to transmission of documents employing publish-subscribe protocols.

While the types of clients described above may be honest but curious, there are circumstances in which clients may be malicious and colluding. For example, a malicious client may receive a document and perform traffic analysis on another client's communication with, for example, a third party to derive information on whether the two share an interest. The malicious client can build statistics on topics that other clients are interested in and how many topics are being sent to a particular client.

Currently deployed publish-subscribe methods and systems target a very limited set of security or privacy requirements (if at all). For example, centralized architectures generally employ a server that is trusted and that further protects against outsiders and client misbehavior through authentication and transport layer security (e.g., SSL/TLS. See Tim Dierks, Eric Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” Internet Engineering Task Force, Request for Comments 5246, August 2008). Similarly, distributed implementations commonly operate in the “fortress model” in which participants are trusted and outsiders are not trusted (See Yair Amir, Cristina Nita-Rotaru, Jonathan Stanton, Gene Tsudik, “Secure Spread: An Integrated Architecture for Secure Group Communication,” IEEE Transactions on Dependable and Secure Computing (TDSC), 2(3): 248-261, (2005)).

The work of Castro and Liskov (See Miguel Castro and Barbara Liskov, “Practical Byzantine Fault Tolerance and Proactive Recovery,” ACM Trans, Comput. Syst., 20(4): 398-461 (2002)) even as extended to achieve perform well when under attack as described in Yair Amir, Brian Coan, Jonathan Kirsch, John Lane, “Byzantine Replication Under Attack,” In Proc. of the 38th IEEE International Conference on Dependable Systems and Networks (DSN08), 2008: 197-206 and in Allen Clement, Edmund Wong, Lorenzo Alvisi, Mike Dahlin, Mirco Marchetti, “Making Byzantine Fault Tolerant Systems Tolerate Byzantine Faults,” In Proc. of the 6th USENIX Symposium on Networked Systems Design and Implementation, 2009: 153-168, provide functionality in the presence of compromised components, but do not attempt to provide client privacy. A well-studied area in cryptography research, known as Secure Multi-Party Computation (or Secure Function Evaluation (see Andrew Chi-Chih Yao, “Theory and Applications of Trapdoor Functions (Extended Abstract),” In Proc. of IEEE FOCS 1982: 80-91 and Oded Goldreich, Silvio Micali, Avi Wigderson, “How to Play any Mental Game or A Completeness Theorem for Protocols with Honest Majority,” In Proc. of ACM STOC 1987: 218-229) address the general problem of two or more parties, each with its own input, jointly and privately computing a function over the inputs. This general approach provides more capability than is needed to implement private publish-subscribe, and is thus too expensive. Basic and well-studied problems in cryptography research, addressing secure computation of specific functions, include Private Information Retrieval (where a client is interested in obtaining one out of a server's many strings without revealing which one) (see Benny Chor, Eyal Kushilevitz, Oded Goldreich, Madhu Sudan, “Private Information Retrieval,” In J. ACM 45(6): 965-981 (1998) and Eyal Kushilevitz, Rafail Ostrovsky, “Replication is NOT Needed: SINGLE Database, Computationally-Private Information Retrieval,” In Proc. of IEEE FOCS 1997: 364-373), Oblivious Transfer (here, the server transfers the client's desired string without knowing which one or revealing all other ones (see Michael O. Rabin, “How to Exchange Secrets with Oblivious Transfer,” Technical Report TR-81, Aiken Computation Lab, Harvard University, 1981), Private Set Intersection (see, e.g., Michael J. Freedman, Kobbi Nissim, Benny Pinkas, “Efficient Private Matching and Set Intersection,” In Proc. of EUROCRYPT 2004: 1-19 (in this method, two parties hold a set of values and at the end of the protocol one of them can compute the intersection of the two sets), and Conditional Oblivious Transfer in Giovanni Di Crescenzo, Rafail Ostrovsky, Sivaramakrishnan Rajagopalan, “Conditional Oblivious Transfer and Timed-Release Encryption,” In Proc. of EUROCRYPT 1999: 74-89 (a variant of oblivious transfer such that a message is sent from a sender to a receiver if and only if a predicate over the two parties' inputs is true, and the sender does not know the predicate value (hereinafter “COT”).

Other security and cryptography research has directly considered the problem of designing secure and/or private publish-subscribe protocols. This research has fallen short as having either a different participant model (i.e., which typically considers publishers as active participants or entirely distributed models with no servers or third parties), having a different set of capabilities and functionalities (i.e., which typically ignores protocol dynamics such as subscription updates or only target sophisticated filtering rules for content publication), or having a different set of security and/or privacy requirements (i.e., which often requires privacy against intermediate routing nodes or privacy only against the server, or which targets more demanding requirements which ultimately result in non-efficient protocols).

The work described in Costin Raiciu, David S. Rosenblum, “Enabling Confidentiality in Content-Based Publish/Subscribe Infrastructures,” In Proc. of SecureComm 2006: 1-11 (based on ideas on searchable encryption from Dawn Song, David Wagner, and Adrian Perrig, “Practical Techniques for Searches on Encrypted Data,” In Proc. of the IEEE Symposium on Security and Privacy, 2000), provides a very efficient publish-subscribe protocol in a restricted participant model (a 1-server, 1-client model), but which only supports privacy against a server and not against clients and does not support subscription updates by clients and related privacy requirements.

Further, there are circumstances in which both the clients and the server may be malicious (e.g., they may arbitrarily deviate from their protocol) and colluding (i.e., some clients can collude among themselves, or with the server) in their attempt to violate privacy requirements.

Accordingly, what would be desirable, but has not yet been provided, is a method and system for providing security and privacy guarantees in a publish-subscribe protocol in the presence of malicious and colluding servers and clients.

SUMMARY OF THE INVENTION

The above-described problems are addressed and a technical solution is achieved in the art by providing a method for providing security and privacy guarantees in a publish-subscribe protocol in the presence of malicious and colluding participants. In an embodiment, a server transmits to a client a public key. The server receives from the client a pseudonym of an interest based on a division malleable commitment method applied to the public key, wherein the pseudonym of the interest functions as a commitment of the client. The server encrypts an item with a padded key and encrypting the padded key. The server transmits to the client the encrypted item and a pseudonym of a topic associated with the item based on a modification of the commitment by the server using a hybrid conditional-oblivious transfer protocol. When the interest of the client equals the topic associated with the item, the client retrieves a correct padded key to decrypt the encrypted data item; otherwise the client retrieves a random key that is unable to decrypt the encrypted data item.

In an embodiment, the received pseudonym of an interest based on the division malleable commitment method may be an El-Gamal encryption of the interest. The interest pseudonym may be employed by the server to encrypt, during the publish phase a padded key based on a hybrid conditional oblivious transfer sub-protocol. A predicate of the sub-protocol may be further based on an equality predicate. In an embodiment, the sub-protocol may be varied such that the client publishes the interest pseudonym as a public key that is accessible by the server. This implies that the sub-protocol may be a distributed protocol with n participants, where n is at least two, and wherein the server and the client are two of the at least two participants. In an embodiment, after the n participants publish respective interest pseudonyms, a participant that is currently a client may become a publisher.

In an embodiment, the server may receive from the client an add interest or delete interest label. In an embodiment, the server may receive an item with associated topics.

In an embodiment, the server may encrypt an item with a padded key and encrypting the padded key. In an embodiment, the server may encrypt the item with the padded key and encrypts the padded key with the stored symmetric key. The server may transmit to the client the item encrypted with the padded key and the padded key encrypted with the stored symmetric key.

In an embodiment, the server may obtain a stored symmetric key. The server receiving a public key, receiving a pseudonym of an interest, encrypting an item, and transmitting the encrypted item may be performed for each of a plurality of clients when the serve is in a push mode and for a requesting client when the client is in a pull mode.

In an embodiment, the modification of the commitment by the server may be considered a commitment of the server to the interest divided by the topic. The server transmitting the pseudonym of the topic associated with the item based on a modification of the commitment may comprise transmitting a value based on a modification of the public key and a modification of the commitment of the server to the interest divided by the topic.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more readily understood from the detailed description of an exemplary embodiment presented below considered in conjunction with the attached drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram of a system in which embodiments of the present disclosure may operate;

FIG. 2 is a block diagram of a software architecture in which embodiments of the present disclosure may operate;

FIG. 3 is a message sequence diagram illustrating messages exchanged between a client and a server for implementing one embodiment of a publish-subscribe protocol with privacy;

FIG. 4 is a process flow diagram illustrating of one embodiment of a method for providing privacy in a publish-subscribe protocol from the point of view of the server;

FIG. 5 is a process flow diagram illustrating of one embodiment of a method for providing privacy in a publish-subscribe protocol from the point of view of a client; and

FIG. 6 illustrates a diagrammatic representation of a machine in the example form of a computer system within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed.

It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and system for providing security and privacy guarantees in a publish-subscribe protocol in the presence malicious and colluding servers and clients.

In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.

FIG. 1 is a block diagram of a system 100 in which embodiments of the present disclosure may operate. The system 100 realizes a publish-subscribe protocol with privacy based on the following basic definitions about data, participants, and communication capabilities between the participants. As used herein, items are digital documents in any format to be published. A dictionary is a collection of all topics that each item may or may not relate to, and is known to all participants (e.g., all 128-character strings). Interests are elements from the dictionary associated with a client. Topics are elements from the dictionary associated with an item to be published.

Returning to FIG. 1, the system 100 includes a server 102 configured to process submitted items (and associated topics) and interests of one or more clients 104 a-104 n to realize the publish-subscribe functionality. In an embodiment, the server 102 functions as a publisher. The one or more clients 104 a-104 n are configured to submit and update subscriptions based on their interests and configured to receive items that match their current interests. In an embodiment, the clients 104 a-104 n function as subscribers. The server 102 and the clients 104 a-104 n may be interconnected by a network 106 (e.g., the Internet). In one example, the network 106 may be assumed to encounter no packet loss or temporarily disconnected participants 102, 104 a-104 n.

In one embodiment, the system 100 is configured to implement a publish-subscribe protocol by employing phases of operation comprising setup, subscription, publication, and optionally, item deletion. During the setup phase, the server 102 and the clients 104 a-104 n may exchange messages to initialize their data structures and/or cryptographic keys. During the subscription phase, the clients 104 a-104 n may update (add/delete) their interests with the server 102. During the publication phase, after receiving a new item (e.g., a data item) and associated topics, the server 102 may distribute the item to the clients 104 a-104 n based on the topics of the item and the interests of the clients 104 a-104 n. Publication protocols may follow at least one of two modes: push mode and/or pull mode. In push mode, after an item is submitted, the item is processed by the server 102 and transmitted to one or more of the clients 104 a-104 n. In pull mode, at any given time, a client (e.g., 104 a) may query the server 102 for any (not previously retrieved) item whose topics match the interests of the client (e.g., 104 a). In the item delete phase, the server 102 may delete items (e.g., for storage efficiency purposes).

A person skilled in the art would appreciate that the (data) item input to the server 102 may originate from any number and types of sources. For example, the (data) item may originate from the publisher/server 102. In another example, the publisher may receive the next data item from any other party that asks the publisher to publish a data item. In another example, the publisher may receive the next (data) item from an independent information source.

FIG. 2 is a block diagram of a software architecture 200 in which embodiments of the present disclosure may operate. Referring now to FIGS. 1 and 2, each of the server 102 and the clients 104 a-104 n is configured to implement corresponding processing logic 202, 204, each of which implements corresponding encryption logic 206, 208 for implementing a “hybrid encryption” method described hereinbelow.

To avoid malicious and colluding behavior on the part of the server 102 and the clients 104 a-104 n, no secret key is shared among the clients 104 a-104 n; otherwise, collusion between clients 104 a-104 n and the server 102 may reveal the secret key to the server 102, thus rendering the key no longer secret. In an embodiment, asymmetric cryptographic primitives may be employed. To minimize potential degradation in efficiency relative employing symmetric keys, in an embodiment, the asymmetric cryptographic primitives may be employed using a “hybrid encryption” method (see e.g., Yvo Desmedt, Rosario Gennaro, Kaoru Kurosawa, Victor Shoup, “A New and Improved Paradigm for Hybrid Encryption Secure Against Chosen-Ciphertext Attack,” J. Cryptology 23(2): 91-120 (2010)), where an asymmetric encryption method may be employed only once per communication session to establish a key for a symmetric encryption scheme used thereafter.

Specifically, a “hybrid conditional oblivious transfer” protocol may be employed where the predicate condition of COT is equality between a pseudonym of a topic of an item and a pseudonym of an interest of a client interest, and where the first of such transfers for a pair (i.e., item topic, client interest) is performed using asymmetric primitives and all subsequent transfers for the same pair re-use the symmetric key established during the first transfer, using memoization. In one embodiment, a hybrid COT may be employed for the matching predicate by running in parallel a hybrid COT for the equality predicate for each pair (item topic, client interest). In one embodiment, the hybrid COT for the equality predicate may employ (1) for a symmetric encryption protocol portion, the COT employed in co-pending, commonly-owned, U.S. patent application Ser. No. ______, entitled, “PRIVACY-PRESERVING PUBLISH-SUBSCRIBE PROTOCOL IN A CLOUD-ASSISTED BROADCAST MODEL”, Attorney Docket No. 26534.11, and (2) for an asymmetric encryption protocol portion, a division-malleable commitment scheme employing, for example, El-Gamal encryption as described in Taher ElGamal, “A Public-Key Cryptosystem and a Signature Scheme Based on Discrete Logarithms, “IEEE Transactions on Information Theory 31(4): 469-472, 1985 (hereinafter “El-Gamal”) and an optimized variant of an oblivious transfer protocol as described in Moni Naor, Benny Pinkas, “Efficient Oblivious Transfer Protocols,” In SODA 2001: 448-457 (hereinafter “Naor”).

FIG. 3 is a message sequence diagram 300 illustrating messages exchanged between a client (e.g., 104 a) and the server 102 for implementing one embodiment of a publish-subscribe protocol with privacy. For simplicity, one interest and one topic are assumed. Since privacy against malicious clients is desired, communication between all participants is secured using methods for encryption, authentication and time-stamping that achieve the needed security properties without incurring the full cost of SSL/TLS (Secure Sockets Layer/Transport Layer Security).

During a setup phase 302, in one embodiment, the client (e.g., 104 a) may obtain from the server 102 a public key needed for a computation of a division-malleable commitment method (e.g., El-Gamal). This key is the same for all clients 104 a-104 n and is computed as a triple (p, q, g), where p, q are primes such that p=2q+1 and g is a generator of the subgroup Z_(p) of order q. The client (e.g., 104 a) only accepts this key if it verifies that p, q are primes such that p=2q+1 and g is a generator of the subgroup of Z_(p) of order q. The

During a subscription phase 304, for a client (e.g., 104 a) to add an interest to a current subscription, the client 104 a obtains a pseudonym, ip, for an interest x to be added by computing an integer h=g_(w) mod p and an El-Gamal encryption (u, v)=(g_(r) mod p, h_(r) x mod p), for random r, p in Z_(q). The client (e.g., 104 a) transmits the interest pseudonym triple ip=(h, u, v) to the server 102. This can be viewed as a division-malleable commitment, in that by dividing the third value, h_(r) x mod p, by some integer y in Z_(p), one obtains a commitment to x/y mod p. This triple is may be transmitted by the client (e.g., 104 a) to the server 102 with an “add” label. When this protocol is employed to delete an interest to the current subscription, the client (e.g., 104 a) retrieves from its storage the previously used triple and transmits the triple to the server 102 with a “delete” label. The server 102 adds/deletes the interest to/from the current subscription of the client (e.g., 104 a).

During a publish phase 306, in an embodiment, the server 102 may run an equality-based-COT protocol with a client (e.g., 104 a) requesting a pull or for each of the clients 104 a-104 n in push mode. For each topic pseudonym tp and each interest pseudonym ip, the server 102 and the client (e.g., 104 a) may run a hybrid COT sub-protocol to transmit an encrypted item's decryption key based on the equality predicate “tp=ip”.

In an embodiment, the sub-protocol may be implemented as follows:

(a) If the sub-protocol was previously executed, then the server 102 obtains an associated stored symmetric key skey and transmits to the client c and z such that c=E_(skey)(k|pad) and z=E_(k) (item).

(b) Otherwise, the server 102 transmits a single message to the client (e.g., 104 a), computed as follows: for each item's topic y, the server 102 modifies a commitment (h, u, v) of the client (e.g., 104 a) to interest x so that it becomes a commitment (h, u, v/y) to x/y; then, the server 102 computes two values (d, e)=(g_(a) u_(b) mod p, h_(a) (v/y)_(b) k mod p) where k is the (padded) key used to encrypt the item and a, b are randomly chosen from Z_(q), and transmits the two values (d, e) to the client (e.g., 104 a).

(c) The client checks if e/d_(w) mod p is a padded key k; if so, it uses k to decrypt the encrypted item z. Padding may be added to messages from the server 102 to all of the clients 104 a-104 n to avoid traffic analysis that may reveal the number of a client's interests to other malicious clients.

More particularly, when (1) a client's interest x is=data item's topic y, the client (e.g., 104 a) retrieves the correct key k to decrypt the encrypted data item. When (2) the client's interest x is different from data item's topic y, the client (e.g., 104 b) retrieves some key k′, but this key is random and thus very different from the key k that may be employed to decrypt the encrypted data item.

To observer that both properties are true, observe that the client's computation e/d̂w mod p above for Publish, step (c), may be written as (ignoring mod p for simplicity):

$\begin{matrix} {{e/{d\hat{}w}} = {e*{d\hat{}\left\{ {- w} \right\}}}} \\ {= {\left( {{h\hat{}a}*{\left( {v/y} \right)\hat{}b}*k} \right)*{\left( {{g\hat{}a}*{u\hat{}b}} \right)\hat{}\left\{ {- w} \right\}}}} \\ {= {\left( {{h\hat{}a}*{\left( {{h\hat{}r}*\left( {x/y} \right)} \right)\hat{}b}*k} \right)*{\left( {{g\hat{}a}*{g\hat{}\left\{ {rb} \right\}}} \right)\hat{}\left\{ {- w} \right\}}}} \\ {= {{g\hat{}\left\{ {aw} \right\}}*\left( {{\left( {{g\hat{}\left\{ {rw} \right\}}*\left( {x/y} \right)} \right)\hat{}b}*k*{g\hat{}\left\{ {- {aw}} \right\}}*{g\hat{}\left\{ {- {wrb}} \right\}}} \right.}} \\ {= {{\left( {x/y} \right)\hat{}b}*k}} \end{matrix}$

where the first and last equality follow from algebraic simplifications, the second equality follows from the expression of e and d (as in step (b)), the third equality follows from the expression of u and v (as in Subscribe), and the fourth equality follows from the expression of h (as in Subscribe).

Now, to see that property (1) is true, when x=y, e/d̂w=1̂b*k=k (i.e., the key k that encrypts the data item). Finally, to see that property (2) is true, when x is different from y, then e/d̂w=k′, for some random k′ (since b is random).

In an embodiment, the subscribe sub-protocol may be varied such that a client (e.g., 104 a) may publish an interest pseudonym as a public key that is accessible by the server 102. This implies that in a distributed protocol with n participants, after the n participants publish their interest pseudonyms, any party can later act as a publisher.

FIG. 4 is a process flow diagram illustrating of one embodiment of a method 400 for providing privacy in a publish-subscribe protocol from the point of view of the server 102. Method 400 may be performed by the processing logic 202 of the server 102 (e.g., in computer system 600 of FIG. 6) that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), firmware, or a combination thereof. In one embodiment, method 400 is performed by the encryption logic 206 of the processing logic 202 of the server 102 of FIG. 2.

In one embodiment, method 400 begins when, during a setup phase 302, at block 405, the server 102 transmits to a client (e.g. 104 a) a public key. The public key may be the same key for all clients 104 a-104 n.

During the subscribe phase 304, at block 410, the server 102 receives from the client (e.g., 104 a) a pseudonym of an interest based on a division malleable commitment method applied to the public key, wherein the interest pseudonym functions as a commitment of the client. The received pseudonym of an interest based on the division malleable commitment method may be an El-Gamal encryption of the interest. The public key may be computed by the server 102 as a triple (p, q, g), where p, q are primes such that p=2q+1 and g is a generator of a subgroup Z_(p) of order q. The client (e.g., 104 a) may be configured to accept the public key when the client verifies that p, q are primes such that p=2q+1 and g is a generator of the subgroup of Z_(p) of order q. The pseudonym of the interest may be an interest pseudonym triple ip=(h, u, v) created by the client by computing an integer h=g_(w) mod p and the El-Gamal encryption (u, v)=(g_(r) mod p, h_(r) x mod p), for random r, p in a subgroup Z_(q). The interest pseudonym may be employed by the server 102 to encrypt, during the publish phase a padded key based on a hybrid conditional oblivious transfer sub-protocol. A predicate of the sub-protocol may be further based on an equality predicate. In an embodiment, the sub-protocol may be varied such that the client (e.g., 104 a) publishes the interest pseudonym as a public key that is accessible by the server 102. This implies that the sub-protocol may be a distributed protocol with n participants, where n is at least two, and wherein the server 102 and the client (e.g., 104 a) are two of the at least two participants. In an embodiment, after the n participants publish respective interest pseudonyms, a participant that is currently a client may become a publisher.

In an embodiment, the server 102 may receive from the client (e.g., 104 a) an add interest or delete interest label.

During the publish phase 306, the server 102 receives an item with associated topics. At block 415, the server 102 encrypts an item with a padded key and encrypting the padded key. More particularly, the server 102 encrypts the item with the padded key and encrypts the padded key with the stored symmetric key. The server 102 transmits to the client (e.g., 104 a) the item encrypted with the padded key and the padded key encrypted with the stored symmetric key.

At block 420, the server 102 transmits to the client (e.g., 104 a) the encrypted item and a pseudonym of a topic associated with the item based on a modification of the commitment by the server 102 using a hybrid conditional-oblivious transfer protocol. In an embodiment, the server 102 obtains a stored symmetric key. The server 102 receiving a public key, receiving a pseudonym of an interest, encrypting an item, and transmitting the encrypted item are performed for each of a plurality of clients 104 a-104 n when the server 102 is in a push mode and for a requesting client (e.g., 104 a) when the client (e.g., 104 a) is in a pull mode.

In an embodiment, the modification of the commitment by the server 102 may be considered a commitment of the server 102 to the interest divided by the topic. The server 102 transmitting the pseudonym of the topic associated with the item based on a modification of the commitment may comprise transmitting a value based on a modification of the public key and a modification of the commitment of the server 102 to the interest divided by the topic. More particularly, the value based on a modification of the public key and the modification of the commitment of the server 102 to the interest divided by the topic may be based on computing two values (d, e)=(g_(a) u_(b) mod p, h_(a) (v/y)_(b) k mod p) where k is the padded key used to encrypt the item and a, b are randomly chosen from a subgroup Z_(q).

In an embodiment, when the interest of the client (e.g., 104 a) equals the topic associated with the item, then the client (e.g., 104 a) is configured to retrieve a correct padded key to decrypt the encrypted item. When the interest of the client (e.g., 104 a) does not equal the topic associated with the item, the client (e.g., 104 a) is configured to retrieve a random key that is unable to decrypt the encrypted item. More particularly, the client (e.g., 104 a) is configured to determine if e/d_(w) mod p is a padded key k and, if so, employs k to decrypt the encrypted item z.

FIG. 5 is a process flow diagram illustrating of one embodiment of a method 500 for providing privacy in a publish-subscribe protocol from the point of view of a client (e.g. 104 a). Method 500 may be performed by the processing logic 204 of the client 104 a (e.g., in computer system 600 of FIG. 6) that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), firmware, or a combination thereof. In one embodiment, method 500 is performed by the encryption logic 208 of the processing logic 204 of the client 104 a of FIG. 2.

In one embodiment, method 500 begins when, during a setup phase 302, at block 505, the client (e.g., 104 a) receives from the server 102 a public key. The client (e.g., 104 a) may accept the public key when the client (e.g., 104 a) verifies that p, q are primes such that p=2q+1 and g is a generator of the subgroup of Z_(p) of order q.

At block 510, the client (e.g., 104 a) transmits a pseudonym of an interest based on the division malleable commitment method applied to the public key. The pseudonym of the interest may be based on the division malleable commitment method comprises transmitting to the server 102 an El-Gamal encryption of the interest. More particularly, the pseudonym of the interest may be an interest pseudonym triple ip=(h, u, v) created by the client by computing an integer h=g_(w) mod p and the El-Gamal encryption (u, v)=(g_(r) mod p, h_(r) x mod p), for random r, p in a subgroup Z_(q). The public key may be computed as a triple (p, q, g), where p, q are primes such that p=2q+1 and g is a generator of a subgroup Z_(p) of order q.

At block 515, the client (e.g., 104 a) receives from the server 102 an encrypted item and a pseudonym of a topic associated with the item based on a hybrid conditional-oblivious transfer protocol. More particularly, the pseudonym of a topic may be based on two values (d, e)=(g_(a) u_(b) mod p, h_(a) (v/y)_(b) k mod p) where k is the padded key used to encrypt the item and a, b are randomly chosen from a subgroup Z_(q). At block 520, the client (e.g., 104 a) determines whether the interest of the client (e.g., 104 a) equals the topic associated with the item. When the interest of the client (e.g., 104 a) equals the topic associated with the item, at block 525, the client (e.g., 104 a) retrieves a correct padded key to decrypt the encrypted data item. When the interest of the client (e.g., 104 a) does not equal the topic associated with the item, at block 530, the client (e.g., 104 a) retrieve a random key that is unable to decrypt the encrypted data item. More particularly, the client (e.g., 104 a) is configured to determine if e/d_(w) mod p is a padded key k and, if so, employs k to decrypt the encrypted item z.

Embodiments of the present disclosure have many advantages over prior art publish-subscribe privacy protection methods. The server 102 can store interests (subscriptions) of many clients 104 a-104 n, add and delete subscriptions dynamically with no need to reprocess all items, and periodically delete items to reclaim space without any participant learning the identity or content of deleted items after they have been deleted. When the server 102 processes an item with a topic that the subscriber had subscribed to, in each COT for the equality predicate, it holds that x=y. Then, by the properties of COT, the client (e.g., 104 a) recovers a padded key k to decrypt the encrypted item.

Even a malicious server colluding with some clients learns no information about interests of any other client, since the interests are transmitted in committed form by the client (e.g., 104 a), where the commitment is based on a public key (p, q, g) transmitted by the server 102 and verified to be correct by the client (e.g., 104 a). Malicious and colluding clients, even if colluding with a server, do not learn any information about other clients' subscriptions, since the distribution of the communication exchanged between the clients 104 a-104 n and the server 102 reveals no information to the clients 104 a-104 n. This is the case even when performing eavesdropping (since communication is secured using SSL/TLS or similar methods), or traffic analysis (message length is the same regardless of the client subscription, assuming padding techniques are employed to mask the number of client interests).

As a consequence of the properties of COT (specifically, that when predicate is false, the message transferred through the COT will not be successfully decrypted by a client (e.g., 104 a)), no information about the key k encrypting an unpublished item is revealed to any participants having no interest in this item during the publication protocol. This follows from the analogue oblivious transfer property of the used scheme from Naor. As a further consequence of the properties of COT (specifically, the server 102 does not realize whether the message transferred through the COT is successfully decrypted by a client (e.g., 104 a)), the server 102 does not learn whether any individual item was published or not. Again, this follows from the analogue oblivious transfer property of the used scheme from Naor.

Publishing each item in the pull mode requires, for the asymmetric cryptography portion, 4 modular exponentiations per (topic, interest) pair, of which 3 can be performed off-line and 1 has to be performed on-line; and for the symmetric cryptography portion, 4 block cipher applications per pair (item topic, client interest). In the push mode, these numbers are multiplied by the number of clients 104 a-104 n. Publication also requires 1 item encryption by the server 102, and 1 item decryption by each interested client 104 a-104 n.

FIG. 6 illustrates a diagrammatic representation of a machine in the exemplary form of a computer system 600 within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. In alternative embodiments, the machine may be connected (e.g., networked) to other machines in a local area network (LAN), an intranet, an extranet, or the Internet. The machine may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a personal digital assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

The exemplary computer system 600 includes a processing device 602, a main memory 604 (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) (such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), a static memory 606 (e.g., flash memory, static random access memory (SRAM), etc.), and a data storage device 618, which communicate with each other via a bus 630.

Processing device 602 represents one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processing device may be complex instruction set computing (CISC) microprocessor, reduced instruction set computer (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device 602 may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processing device 602 is configured to execute processing logic 202, 204 for performing the operations and steps discussed herein.

Computer system 600 may further include a network interface device 608. Computer system 600 also may include a video display unit 610 (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device 612 (e.g., a keyboard), a cursor control device 614 (e.g., a mouse), and a signal generation device 616 (e.g., a speaker).

Data storage device 618 may include a machine-readable storage medium (or more specifically a computer-readable storage medium) 620 having one or more sets of instructions (i.e., the processing logic 202, 204) embodying any one or more of the methodologies of functions described herein. The processing logic 202, 204 may also reside, completely or at least partially, within main memory 604 and/or within processing device 602 during execution thereof by computer system 600; main memory 604 and processing device 602 also constituting machine-readable storage media. The processing logic 202, 204 may further be transmitted or received over a network 626 via network interface device 608.

Machine-readable storage medium 620 may also be used to store processing logic 202, 204 persistently. While machine-readable storage medium 620 is shown in an exemplary embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable storage medium” shall also be taken to include any medium that is capable of storing or encoding a set of instruction for execution by the machine and that causes the machine to perform any one or more of the methodologies of the present invention. The term “machine-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, and optical and magnetic media.

The components and other features described herein can be implemented as discrete hardware components or integrated in the functionality of hardware components such as ASICs, FPGAs, DSPs or similar devices. In addition, these components can be implemented as firmware or functional circuitry within hardware devices. Further, these components can be implemented in any combination of hardware devices and software components.

Some portions of the detailed descriptions are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, as apparent from the above discussion, it is appreciated that throughout the description, discussions utilizing terms such as “enabling”, “transmitting”, “requesting”, “identifying”, “querying”, “retrieving”, “forwarding”, “determining”, “passing”, “processing”, “disabling”, or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

Embodiments of the present invention also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, flash memory devices including universal serial bus (USB) storage devices (e.g., USB key devices) or any type of media suitable for storing electronic instructions, each of which may be coupled to a computer system bus.

The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems may be used with programs in accordance with the teachings herein or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will be apparent from the description above. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 

What is claimed is:
 1. A computer-implemented method for providing privacy in a publish-subscribe protocol, comprising the steps of: transmitting, from a server to a client, a public key; receiving, by the server from the client, a pseudonym of an interest based on a division malleable commitment method applied to the public key, wherein the pseudonym of the interest functions as a commitment of the client; encrypting, by the server, an item with a padded key and encrypting the padded key; and transmitting, by the server to the client, the encrypted item and a pseudonym of a topic associated with the item based on a modification of the commitment by the server using a hybrid conditional-oblivious transfer protocol.
 2. The method of claim 1, wherein when the interest of the client equals the topic associated with the item, then the client is configured to retrieve a correct padded key to decrypt the encrypted data item.
 3. The method of claim 1, wherein when the interest of the client does not equal the topic associated with the item, the client is configured to retrieve a random key that is unable to decrypt the encrypted data item.
 4. The method of claim 1, further comprising encrypting, by the server to the client, the encrypted padded key based on a hybrid conditional oblivious transfer sub-protocol.
 5. The method of claim 4, wherein a predicate of the sub-protocol is further based on an equality predicate.
 6. The method of claim 4, wherein the sub-protocol is varied such that the client publishes the interest pseudonym as a public key that is accessible by the server.
 7. The method of claim 6, wherein the sub-protocol is a distributed protocol with n participants, where n is at least two, and wherein the server and the client are two of the at least two participants.
 8. The method of claim 7, wherein, after the n participants publish respective interest pseudonyms, a participant that is currently a client becomes a publisher.
 9. The method of claim 1, wherein the modification of the commitment by the server is a commitment of the server to the interest divided by the topic.
 10. The method of claim 1, wherein the public key is the same for all clients.
 11. The method of claim 1, wherein transmitting the pseudonym of the topic associated with the item based on a modification of the commitment comprises transmitting a value based on a modification of the public key and a modification of the commitment of the server to the interest divided by the topic.
 12. The method of claim 1, wherein receiving a pseudonym of an interest based on the division malleable commitment method comprises receiving an El-Gamal encryption of the interest.
 13. The method of claim 1, further comprising, after transmitting the public key, receiving the pseudonym, encrypting the item, and transmitting the encrypted item: obtaining, by the server, a stored symmetric key; encrypting the item with the padded key and encrypting the padded key with the stored symmetric key; and transmitting, by the server to the client, the item encrypted with the padded key and the padded key encrypted with the stored symmetric key.
 14. The method of claim 1, further comprising, transmitting, by the client to the server, an add interest or delete interest label.
 15. The method of claim 1, wherein the transmitting a public key, receiving a pseudonym of an interest, encrypting an item, and transmitting the encrypted item is performed for each of a plurality of clients when the server is in a push mode and for a requesting client when the client is in a pull mode.
 16. The method of claim 1, further comprising receiving, by the server, the item with associated topics.
 17. A non-transitory computer readable storage medium including instructions that, when executed by a client, causes the client to: receive, by a client from a server, a public key; transmit, by the client to the server, a pseudonym of an interest based on the division malleable commitment method applied to the public key; and receive, by the client from the server, an encrypted item and a pseudonym of a topic associated with the item based on a hybrid conditional-oblivious transfer protocol, wherein when the interest of the client equals the topic associated with the item, the client retrieves a correct padded key to decrypt the encrypted data item and wherein when the interest of the client does not equal the topic associated with the item, the client retrieve a random key that is unable to decrypt the encrypted data item.
 18. The non-transitory computer readable storage medium of claim 17, wherein transmitting a pseudonym of an interest based on the division malleable commitment method comprises transmitting to the server an El-Gamal encryption of the interest.
 19. A computer system, comprising: a memory; a processing device, coupled to the memory, the processing device to: transmit, from a server to a client, a public key; receive, by the server from the client, a pseudonym of an interest based on the division malleable commitment method applied to the public key, wherein the pseudonym of the interest functions as a commitment of the client; encrypt, by the server, an item with a padded key and encrypting the padded key; and transmit, by the server to the client, the encrypted item and a pseudonym of a topic associated with the item based on a modification of the commitment by the server using a hybrid conditional-oblivious transfer protocol.
 20. The system of claim 19, wherein when the interest of the client equals the topic associated with the item, then the client is configured to retrieve a correct padded key to decrypt the encrypted data item.
 21. The system of claim 19, wherein when the interest of the client does not equal the topic associated with the item, the client is configured to retrieve a random key that is unable to decrypt the encrypted data item.
 22. The system of claim 19, further comprising encrypting, by the server to the client, the encrypted padded key based on a hybrid conditional oblivious transfer sub-protocol.
 23. The system of claim 19, wherein a predicate of the sub-protocol is further based on an equality predicate.
 24. The system of claim 19, wherein the modification of the commitment by the server is a commitment of the server to the interest divided by the topic.
 25. The system of claim 19, wherein transmitting the pseudonym of the topic associated with the item based on a modification of the commitment comprises transmitting a value based on a modification of the public key and a modification of the commitment of the server to the interest divided by the topic. 